Upgrade Quarkus version to 2.6

As with !213 (merged), we recreated a Quarkus project and carefully inspected what changed. This is mainly the Dockerfiles.

mvn io.quarkus.platform:quarkus-maven-plugin:2.6.3.Final:create \
    -DprojectGroupId=theodolite \
    -DprojectArtifactId=theodolite \
    -DclassName="org.acme.rest.GreetingResource" \
    -Dpath="/theodolite" \
    -Dextensions="kotlin,kubernetes-client" \
    -DbuildTool=gradle

Might fix #328 (closed).